Washington Street West watershed - City of Champaign

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Draft ReportWest Washington Street Watershed Master PlanThe depression storage variables were determined using the default methods within thesoftware. The impervious area variables were calculated using the equationDp=0.0303*S^(-0.49), where the average catchment slope (S), in percent, was assumedto be 2% throughout the watershed for an impervious depression storage of 0.021. Thepervious depression storage for grassed urban surfaces is typically 0.1 inches. TheManning’s “n” values were determined using values from the imbedded tables within thesoftware, resulting in an impervious Manning’s “n” value of 0.014 and a pervious valueof 0.3. The zero detention (%), the portion of the watershed that drains directly to thestorm sewers, was lowered from the default of 25% to a more typical value of 20%.The Copper Slough XP-SWMM model used the SCS method for infiltration calculations.The curve numbers were between 73 and 95, with corresponding times of concentration.This method is useful for large watersheds or when soil data is unavailable, but the WestWashington Street watershed has soil type data readily available and therefore makes theGreen Ampt method a more accurate runoff determination method. The ranges ofvariables used in the Green Ampt method are typical of type B soils for the SCS method,which are used in the Copper Slough XP-SWMM model.5.2 DESIGN STORMS AND PEAK FLOW ESTIMATIONSynthetic (design) storms were used to predict peak flows throughout the watershed underexisting conditions and to model proposed improvements. The West Washington Street peakflow rates were evaluated using the 2-hour storm with the following return intervals: 50-year, 25-year, 10-year, 5-year, and 1-year. The 2-hour duration was the critical duration storm, having themost impact on high water levels throughout the system, and was used in the flooding analysis.The trunk storm sewers were evaluated using the 10-year storm, consistent with the City’s currentdesign criteria (many of the existing trunk sewers were built prior to the 10-year design standard).Time distributions for rainfall (Huff curves) were used as published in “Bulletin 71 - RainfallFrequency Atlas of the Midwest” (Midwestern Climate Center and Illinois State Water Survey,1992). The Huff 1 st Quartile distribution was used for the storms. Rainfall depths are shown foreach design storm in Table 5-1.Table 5-1Rainfall depths (in) for typical storms at Urbana, IllinoisSource: Bulletin 70Storm FrequencyStormDuration 50 year 25 year 10 year 5 year 1 year2 hour 3.22 2.86 2.48 2.18 1.522-hour 3.44 2.97 2.52 2.19 1.46The top row in Table 5-1 represents the precipitation data for the City of Urbana in the pointrainfall tables of Bulletin 70 and the bottom row displays the rainfall data for the Eastern regionof Illinois in the area tables of Bulletin 70. The area tables may produce a more conservativerainfall depth for modeling; however, the area tables provide an average rainfall depth for aSeptember 20095-3Clark Dietz, Inc.
Draft ReportWest Washington Street Watershed Master Planseveral county area and are normally used for large watersheds. The point rainfall tables aretraditionally used for smaller watersheds that are located near one of the cities listed in the table.The rainfall data at Urbana in the point rainfall tables better represents the local simulated rainfalldepths for this watershed. Since the difference in rainfall depths is minor and the point rainfalltable data is traditionally used for small watersheds, the point rainfall table data (row 1 in Table5-1) will be used in this study.5.2.1 Downstream Water ElevationsThe existing conditions downstream water elevation is modeled as a free outlet, whichmeans that there is no flooding or flow restrictions at the downstream point in the model.This condition typically does not exist for larger storm events and often requires anincrease in tailwater conditions to coincide with flooding along the channel. Thedownstream outlet conditions are located a few miles downstream of the WestWashington Street trunk storm sewer outlet and therefore have very little impact on thehydraulic conditions in the channel at this point. The water elevation near the seweroutlet likely represents an accurate water level in the Copper Slough for the design stormbeing simulated. There are no required changes to variables between model runs otherthan the storm type to set the tailwater conditions in the Copper Slough at the WestWashington Street storm sewer outlet.5.3 EXISTING CONDITIONS: KEY FINDINGSThe Existing Conditions XP-SWMM model was used to identify where undersized sewers mayexist. The 10-year recurrence interval storm (2-hour duration) was used to evaluate existingstorm sewers for surface flooding potential. The 50-year storm was used to evaluate the impacts(locations and depth) of surface flooding routing.The 2-hour duration storm was chosen as it represents the critical duration for a watershed of thissize. For smaller urbanized watersheds (less than one square mile), the 2-hour duration rainfalltypically generates the highest peak flow calculations.5.3.1 Undersized Trunk Storm SewersTrunk storm sewers are defined as undersized where the Existing Conditions XP-SWMMmodel predicted one or both of the following scenarios (under 10-year storm conditions):• The slope of the hydraulic grade line exceeds the slope of the sewer• The hydraulic grade line exceeds the ground surface, indicating floodingpotentialFigure 5-2 illustrates the locations of all trunk sewers modeled as part of this study.Undersized trunk storm sewers are identified below. The following text describes theindividual problems associated with the undersized trunk sewer:West Washington Street: Between North Russell Street and the Copper Slough, the 36and 42-inch diameter trunk storm sewer north of West Washington Street (see Figure 5-2) is undersized. The majority of flow (for the 10-year storm) is conveyed west viaunofficial surface flow or stored in localized depressions along the West WashingtonStreet corridor.September 20095-4Clark Dietz, Inc.
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Washington Street West watershed - City of Champaign

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